Yes, you can. But why you would do something like that?
Performance depends on set up you go with. If we go back to your previous question How to replicate this architecture in DO for drupal?, we can see that there was talk about 2 web servers behind load balancer.
Possible set ups could be:
1) Make one more node for files w/ block storage and host attach it on 2 other servers.
I don’t see much benefit with this. You’ll be on the same, as you will still get that from one server. You use two web servers, yes, but data is still from one so if you have very high traffic, that can be potential problem.
2) One web server with data on another.
Nothing too much, you’ll just slow it down (unnoticeable, but still slower).
3) Two web servers, data on third, attaching and reattaching NFS from server to server.
Data will be available on only one in time so that’s not load balancing.
If you share you exact idea, we can discuss about performance, I got some ideas but there not so efficient in my opinion.
About speed, if you go with same data center, you can setup private networking for NFS. How to set up a new droplet for extended storage only? (Block space n/a yet.) was a questions from a few days ago, it use droplets space instead of block storage, but maybe it will give you idea of speed (read one of comments).
I was not working with Drupal, but as this is CMS, files should be always same. When you do changes, just set up some replication system and have files on both servers.
Maybe someone have another experience, but this is my opinions and I hope it will be helpful to you. =)